1. Field of the Invention
The present invention relates to a deformable mirror which controls a shape of a reflecting surface by using an electrostatic force.
2. Description of the Related Art
A deformable mirror, in which an MEMS (micro electro mechanical system) technology of changing a shape of a reflecting surface by an electrostatic force is used, has hitherto been drawing attention. For controlling the shape of the reflecting surface highly accurately, it is necessary to measure a deformed shape. As a method of measuring the shape of the reflecting surface, a method in which the deformed shape is determined by measuring a change in an electrostatic capacitance as proposed in Japanese Patent Application Laid-open Publication No. 2002-228813 for example, is available.
FIG. 19 shows a structure of a conventional deformable mirror. The conventional deformable mirror includes a piezoelectric control substrate 1 and an upper substrate 5. A flexible thin film 8 having a reflecting surface which is deformed by an electrostatic force of attraction and an upper electrode 6 are formed on the upper substrate 5.
A voltage control circuit 3 and a control electrode 2 which is disposed facing the flexible thin film 8, and which serves as both controlling and for capacity detecting are formed on the piezoelectric control substrate 1. Moreover, external lead electrodes 4 and 7 are formed on these substrates respectively.
The reflecting surface is deformed by an electrostatic driving force which is generated by applying a voltage between the upper electrode 6 and the control electrode 2. An arrangement is made such that it is possible to calculate an amount of deformation of the reflecting surface by calculating a capacitance between the upper electrode 6 and the control electrode 2.
As a structure of a detecting circuit, a structure shown in FIG. 20 is proposed. A high voltage 10a is applied to the control electrode 2 via a resistor. When a high frequency voltage 10b is applied to an electrostatic capacitance detecting electrode 2′, an electric potential of the control electrode 2 is changed. This change in the electric potential is monitored as a change in a current at an electrostatic capacitance detecting circuit 6A through the upper electrode 6. From a phase and amplitude of this current, the change in the electrostatic capacitance, in other words, a displacement of the reflecting surface is detected.
In a conventional electrostatic driving type deformable mirror, a structure in which the control electrode 2 is divided into a plurality of electrodes corresponding to one upper electrode 6 in order to change the shape of the reflecting surface freely, has also been taken into consideration. In this case, the capacitance of the plurality of control electrodes 2 is measured one after another. Moreover, a method of determining one after another, the shape of the corresponding reflecting surface, has been shown.
However, in the deformable mirror with a structure having the plurality of control electrodes as described above, the shape of the reflecting surface has to be determined by measuring one after another, the capacitance for each control electrode. Therefore, sometimes the process takes time. Moreover, although it is difficult to determine continuously, particularly to determine simultaneously, an amount of deformation of the reflecting surface at a plurality of positions, it is important to determine in such manner while deforming at a high speed the shape of the reflecting surface.